The invention relates to the field of selective optical filtering by electrical modulation of the spectral transfer function.
It relates more particularly, although not exclusively, to electrically tunable optoelectronic filter devices intended for the wavelength demultiplexing of the channels of certain optical telecommunication installations, and especially installations using multiplexing systems known as WDM (Wavelength Division Multiplexing) and DWDM (Dense Wavelength Division Multiplexing).
To provide this type of demultiplexing, Fabry-Pxc3xa9rot type filters have been proposed that include a resonant air cavity placed between partial reflectors. The wavelength tunability of these filters is obtained by displacing at least one of the partial reflectors, which is mounted in an elastic manner, under the effect of an electrostatic pressure.
Devices of this type have been described, for example, in the scientific articles mentioned below:
M. S. Wu, G. S. Yuen and C. J. Chang-Hasnain, xe2x80x9cWidely tunable 1.5 xcexcm micromechanical optical filter using AlOx/AlGaAs DBRxe2x80x9d, Electronics Letters 33, 1702 (1997);
P. Tayebati, P. Wang, D. Vakhshoori and R. N. Sacks, xe2x80x9cMicroelectromechanical tunable filters with 0.47 nm linewidth and 70 nm tuning rangexe2x80x9d, Electronics Letters 34, 76 (1998), and xe2x80x9cWidely tunable Fabry-Perot filter using Ga(Al)As-AlOx deformable mirrorsxe2x80x9d, IEE Photonics Technology Letters 10, 394 (1998);
P. Tayebati, P. Wang, A. Azimi, L. Maflah and D Vahshoori, xe2x80x9cMicroelectromechanical tunable filters with stable half symmetric cavityxe2x80x9d, Electronics Letters 34, 1967 (1998);
D. Rondi, R. Blondeau, G. Guillot and P. Viktorovitch, xe2x80x9cHighly selective 1.55 xcexcm InP/Air-gap micromachined Fabry-Perot Filter For Optical Communicationsxe2x80x9d, Electronics Letters 39, 453 (1998); and
A. Spisser, R. Ledantec, C. Seassal, J. L. Leclercq, T. Benyattou, D. Rondi, R. Blondeau, G. Guillot and P. Viktorovitch, xe2x80x9cHighly selective and widely tunable 1.55 xcexca InP/Air-gap micromachined Fabry-Perot Filter For Optical Communicationsxe2x80x9d, IEEE Photonics Technology Letters, 10 (9), 1259 (1998).
However, these devices having a single resonant cavity cannot provide simultaneously, for theoretical reasons, suitable separation (typically greater than xe2x88x9220 dB) between adjacent channels and a useful passband sufficient for transmission, without attenuation, of a rapid modulation (typically greater than 10 GHz) of the light signal.
It is known that suitable filters, that is to say those possessing a more xe2x80x9crectangularxe2x80x9d passband, are obtained using several Fabry-Pxc3xa9rot cavities coupled together. A filter having two coupled cavities makes it possible, for example, to meet the specifications of the current WDM systems for a given wavelength. Solutions have already been proposed for achieving tunability of filters having two coupled cavities.
Thus, document U.S. Pat. No. 5,103,340 proposes the coupling of two resonant air cavities, called xe2x80x9cthickxe2x80x9d cavities because they have optical thicknesses of mxcex/2 and nxcex/2, m and n being integers of around 100 and close enough to have spaced-apart common resonance wavelengths. The thickness of each cavity is controlled here by a common piezoelectric actuator placed so that the displacements of the two cavities are proportional to the integers m and n. However, it is quite difficult to tune the two cavities simultaneously and the overall size of these devices is an obstacle to a high level of integration. In addition, these devices require high control voltages, typically of the order of a few hundred volts, which make them difficult, or even impossible, to use in environments in which the energy consumption levels are low.
The object of the invention is to remedy all or some of the aforementioned drawbacks.
For this purpose, the invention provides an optoelectronic device comprising i) first means that define a first (thick) resonant cavity, the thickness and the composition of which are chosen to offer a multiplicity of resonant transmission modes in a chosen wavelength range, and ii) second means that define a second (thin) resonant cavity, the thickness and the composition of which are chosen to offer a single resonant transmission mode in the chosen wavelength range, the device being characterized in that it also includes means for optically coupling said first and second means, and electrostatic means able to apply an electrical voltage to said second means, said electrical voltage making it possible to vary the thickness of the second cavity and the spectral position of the associated resonant mode so that this mode coincides with any one of the resonant modes of the first cavity (said mode being chosen from all those that it has). The device may thus transmit an incident light wave whose wavelength is that of a resonant mode common to the thin and thick cavities.
In the device according to the, invention, it is sufficient to tune the second cavity to a resonant mode that coincides with a resonant mode of the first cavity, which is much simpler than tuning both cavities simultaneously and can be carried out with simple means. The optical coupling of the two cavities allows transmission of really xe2x80x9crectangularxe2x80x9d signals, in accordance with the standards of the WDM and DWDM systems.
The expression xe2x80x9coptical coupling meansxe2x80x9d is understood here to mean a means allowing optical interaction between the thin and thick cavities. Furthermore, xe2x80x9cany onexe2x80x9d is understood to mean the fact of selecting, according to the requirements, one or other mode of the first cavity by controlling the thickness of the second cavity. In other words, any one of the modes of the first cavity may be chosen, depending on the requirements.
Thus, the two, thin and thick, coupled cavities produce a filter whose transmission wavelength may be adjusted in a discontinuous manner over each (or at least a certain number) of the wavelengths of the various transmission modes of the thick cavity. The spectral characteristic of the transmission function thus produced is that of a filter having two coupled cavities, and therefore has rejection and passband characteristics that are greatly superior to those of a filter with a single cavity.
In one advantageous embodiment, the thickness and the composition of the first (thick) cavity are chosen so that the multiplicity of its resonant transmission modes defines a comb, the position of the modes and the distance between adjacent modes (or intermode spacing) of which are chosen so as to coincide respectively with the position and distance of the wavelengths of the light signal that it is desired to demultiplex, such as those that are defined by the international standards (such as the ITU).
Preferably, the first means that define the first (thick) cavity comprise two approximately parallel partial reflectors spaced apart by a first layer of material (preferably semiconductor material), the thickness of which fixes the position of the resonant modes of the first cavity, and which ensure the resonance of this first cavity.
Also preferably, the second means that define the second (thin) cavity comprise at least two approximately parallel partial reflectors spaced apart by a second layer of material (preferably an air layer), the thickness of which defines the position of the resonant mode of the second cavity, and which ensure the resonance of this second cavity.
According to another feature of the invention, the electrostatic means are produced by electrically connecting each of the two partial mirrors of the second means to a first electrode and a second electrode in such a way that, when a potential difference is applied between said first and second electrodes, the thickness of the air cavity located between the two partial reflectors (or mirrors) changes. In one particular embodiment, the second means are configured so as to define at least one substructure of the pin or nip junction type. In this case, the pin junction, or alternatively the nip junction, is reverse-biased by the electrostatic means.
Advantageously, the partial reflectors are Bragg reflectors consisting of at least one quarter-wave-type alternation of two materials having different refractive indices. These alternations may relate, for example, to silicon and silicon dioxide layers and/or air and semiconductor layers (as described in the articles by Spisser et al. cited in the introduction).
Advantageously, the first and second means are composed of semiconductor materials, and preferably materials of the III-V type, such as gallium arsenide (GaAs), InGaAs or indium phosphide (InP). This makes it possible to use epitaxy and selective etching techniques, especially when the second (thin) layer is an air layer and/or the reflectors that surround it are Bragg-type reflectors formed by alternations of semiconductor layers and air layers (the air optionally being replaced with another fluid materialxe2x80x94a gas or liquidxe2x80x94or a polymer).
In one advantageous embodiment, the means for coupling the thin and thick cavities are third means (for example a layer of material or an air layer) that are interposed between the first and second means (and especially between two reflectors) and having dimensions chosen so as to ensure optimum optical coupling between the first and second cavities. According to the principles of producing Fabry-Pxc3xa9rot cavity filters well known to those skilled in the art (see, for example, H. A. Macleod, xe2x80x9cThin-film optical filtersxe2x80x9d, New York, McGraw-Hill, 1986), the coupling means may be formed, for example, by a layer of the quarter-wave type.
The device may also include means for shifting the frequency of the multiplicity of resonant transmission modes of the first means. Preferably, the frequency shift is obtained by a controlled variation of the temperature of the first means (at least).